This invention relates to molten carbonate fuel cells and more particularly to fuel cells with cathodes based on transition metals. At the present stage of their development, molten carbonate fuel cells typically include porous anode and cathode electrodes with an electrolyte component which separates and provides an ion conducting path between the electrodes. Means are provided for directing a hydrogen-affording fuel gas to the anode and an oxidant gas to the cathode, together with appropriate housing members including means for expelling reaction products from the cell. The electrolyte component is composed of one or more alkali metal carbonates as the electrolyte which is molten at the operating temperature of 600.degree.-700.degree. C. and dispersed in a solid matrix or tile. In the operation of the cell, carbonate ions are catalytically formed at the cathode from carbon dioxide and oxygen. The ions travel through the electrolyte towards the anode. At the anode, hydrogen in the fuel gas reacts with the carbonate ions to form water and carbon dioxide while releasing electrons to the external circuit.
As illustrated in U.S. Pat. No. 3,481,788, special openings or enlarged pores may be provided in the tile for the removal of the water and carbon dioxide by venting these products away from the cell.
Frequently in these cells, the cathode is formed of a nickel based composition such as a lithiated nickel oxide which provides electronic conductivity in addition to the required catalyst activity. The anode may be formed of a porous nickel composition. In operation of the cell, the fuel and oxidant gases may be supplied at atmospheric pressure or at elevated pressures up to about 10 atmospheres. At the higher pressures, the cell potential is increased and the overall operating efficiency of the cell is improved.
One of the problems with molten carbonate fuel cells having nickel based cathodes is associated with the loss or migration of nickel into the electrolyte. Nickel ions migrate through the electrolyte and are reduced by hydrogen gas diffusing into the electrolyte from the anode region to form a metallic nickel deposit within the electrolyte structure. The transfer of the nickel from the cathode is associated with the reaction NiO+CO.sub.2 .fwdarw.NiCO.sub.3. At elevated pressures, the reaction tends to form increased amounts of NiCO.sub.3 for transfer to the electrolyte. Since nickel has been considered to be one of the more promising materials for fuel cells, this problem is of concern in this art and efforts have been made to reduce the importance of the problem.
Accordingly, one of the objects of this invention is a molten carbonate fuel cell with a reduced migration of metal ions and particularly nickel ions from the cathode into the electrolyte. A second object is a molten carbonate fuel cell with a reduction in conditions in the electrolyte for reducing metal ions and particularly nickel ions to elemental metal. Another object is a molten carbonate fuel cell with desirable performance at elevated pressures. These and other objects will become apparent from the following description.